Batteries require facile charge transfer to be successful, but it is a challenge when the desired active material is an electronic insulator. This is the case in many types of well-studied battery materials: LiFePO4 in Li-ion batteries, Li2O2 in Li-air batteries, and S8 and Li2S in Li-sulfur batteries. The insulating nature of these materials can cause low rate tolerance, low capacities, and polarization. This has remedied by adding a conductive additive, commonly conductive carbon materials.
Additional challenges occur when the insulating product of battery discharge undergoes a phase change to deposit as a solid. When a solid forms during cycling, the deposition of this species needs to be accounted for to design a successful battery by providing surface area on which it can deposit. In Li-sulfur battery cathodes elemental sulfur (S8) is reduced to Li2S through soluble (in typical battery electrolytes) polysulfide species (Li2Sx, x=4-8), while Li2S, the final discharge product, is an insoluble, electronically insulating species. In Li—O2 batteries, gaseous O2 is reduced to form solid, insulating, insoluble Li2O2. In both cases, the solid phase nucleates on the surface where it is reduced, usually by the conductive carbon additive, and once an insulating layer is formed, the reaction can no longer proceed terminating discharge (although conductivity may be imparted to Li2O2 through Li vacancies). This means that the surface area of the conductive carbon additive contributes to the amount of active material that can be utilized. In order to increase battery capacity, many types of carbon materials with high surface areas are utilized, including commercial microparticles (such as Ketjenblack or Super P), carbon fibers and nanotubes, and hierarchically porous carbons.
A redox mediator is a compound with a reversible redox couple that facilitates electron transfer from the electrode to the active species. Rather than direct electron transfer from the electrode to the active species, electron transfer takes place over two steps; the redox mediator is reduced/oxidized at the electrode, diffuses away, reduces/oxidizes the active species, and in this process is returned to its original state so the process can repeat. Soluble redox mediators have been used in batteries to facilitate the charge, discharge, or both of the Li2O2/O2 cathode in Li—O2 batteries as well to facilitate charge transfer to insulating LiFePO4 in Li-ion batteries. Redox mediators are only beginning to be explored in their application to Li—S batteries, e.g., Aurbach et al. have reported on the use of redox mediators to lower the overpotential required for activating solid-state Li2S cathodes (WO 2015044829). What is needed are new redox mediators and energy storage devices incorporating the new redox mediators. Surprisingly, the present invention meets this and other needs.